Mobile telephones and similar communication devices are rapidly expanding in use and function. Such devices will soon provide Internet access, personal information management, facsimile, and messaging, in addition to telephone communication. This will require a user interface which is more complex, crowded and generally more difficult to use. In addition, electronic devices, such as mobile phones, pagers and the like, are being used in ever expanding situations and environments. Inevitably the devices will be used where only limited light is available, thereby making it even more difficult to operate the user interface. Accordingly effective internal lighting will be an important feature of these devices.
Providing a bright and efficient light source at a reasonable price has become more and more difficult as the devices have been reduced in size and packed with an ever increasing number of features. To accommodate the packaging and cost demands, it is desirable to use a printed circuit board that has its components, including light emitting diodes (LEDs) for illumination, mounted on only one side of the board, referred to as the component side. In many instances the buttons, display and other components of the user interface, which require illumination, are located facing the opposite side of the board. The board therefore will impede the illumination of these components. To resolve this problem, an optical light guide is used to receive light through an opening in the board and bend it to illuminate the desired components. A so called through-the-board light source is constructed to direct the light of an LED through the opening in the printed circuit board.
The relative positioning of the light guide and light sources requires optimized coupling of the components to maximize the distribution of light within the light guide. In the systems of the prior art, as shown in FIGS. 1a and 1b, edge coupling and surface coupling is often used. Each has its limitations, edge coupling works reasonably well when the light guide is thick enough to receive a majority of the light generated by the LED. Since it is desirable to make mobile communications devices thinner, edge coupling is a limitation on design advance. Surface coupling is inherently less efficient because of the need to bend the light which results in the light escaping out of the light guide, as shown in FIG. 1b. Modification of the surface geometry of the light guide to retain more light and reduce losses is attempted, but with only limited success.
A purpose of this invention is, therefore, to provide a lighting system for the user interface components of an electronic device, such as a mobile communication device. More particularly, it is a purpose of this invention to distribute the light from an LED into a light guide with improved efficiency while allowing the thickness of the light guide to be reduced significantly. Another object of this invention is to construct an input diffraction optical element (DOE), such as a grating structure, operatively associated with a light guide to distribute the light from an LED throughout the light guide. It is a further purpose of this invention to use an output diffraction optical element in association with a light guide to transmit light from the light guide to the components of the user interface.
A system for distributing light within a thin light guide is provided using diffraction gratings as a means to optically couple the light from a source, such as a light emitting diode (LED), to the light guide. Planar style light guides have been used to supply light to the user interface of a mobile telephone or other communications device in the past, but the reduction of the thickness of the light guide was limited in order to maintain a reasonable level of coupling efficiency. By the use of diffraction gratings and the like as a coupling mechanism, the light guide can be reduced considerably in thickness while increasing the coupling efficiency.
In the system of this invention the input grating coupling is optimized for each application and the system can include an array of LEDs each having its optimized input grating. By also using diffraction gratings also to out couple the light from the light guide an extremely uniform source of illumination is provided to sensitive user interface components such as a liquid crystal display (LCD)
The pattern of user interface illumination is established for a particular application. This determines the configuration of the light guide, its associated diffraction gratings and the array of LEDs required. A master grating pattern is constructed by means, for example: electron beam lithography and assembled with the light guide in the molding or pressing process of the light guide. In this manner an extremely thin light guide is constructed having increased coupling efficiency.